1. Field of the Invention
The present invention relates to a surface light emission device, a method of manufacturing the same, and a liquid crystal display device, and more specifically, to the structure of a surface light emission device that is preferably used as a front light of a liquid crystal display device.
2. Description of the Related Art
Reflection type liquid crystal display devices that display an image using sun light and illuminating light as a light source are used in mobile phones and mobile information terminals because their power consumption is small. However, the reflection type liquid crystal display device has a problem that it is very poor at a display capability in a dark place where external light is not available. To solve this problem, there are known semi-transmission/reflection type liquid crystal display devices which are arranged such that a back light is disposed to the reflection type liquid crystal display devices so that an image can be displayed in a light transmission mode by tuning on the back light at a location where external light is not available.
However, it is very difficult for the semi-transmission/reflection type liquid crystal display device to make the brightness of display in a semi-transmission mode compatible with the brightness of display in a reflection mode because it is necessary to pass the light from the back light through a reflection film composed of a metal thin film.
To cope with this problem, there have been developed liquid crystal display devices provided with a surface light emission device (front light). In this liquid crystal display device, light from a light source such as a cold-cathode tube which is introduced from a side end surface of a light introduction plate is caused to outgo from a surface thereof and illuminates the liquid crystal display device from the front surface thereof. The provision of the surface light emission device in front of the front surface of the liquid crystal display device permits an image to be displayed in a reflection mode even in a dark place similarly to a case in which external light is used.
FIG. 15 is a structural view in cross section showing an example of a liquid crystal display device having a front light in front of a liquid crystal display unit. In the figure, the liquid crystal display device 100 comprises the front light 110 and the liquid crystal display unit 120. The front light 110 comprises a light introduction plate 112 which is composed of a transparent acrylic resin and a light source 113 which is composed of a cold-cathode tube or a LED (light emitting diode) and disposed to a side end surface 112a of the light introduction plate 112. A plurality of grooves 114 are continuously formed in a stripe shape on the opposing surface 112c of the light introduction plate 112 which is opposite to the light outgoing surface 112b thereof. Each of the grooves 114 is composed of two slanting surfaces which are formed to slant with respect to the light outgoing surface 112b. 
The two slanting surfaces are composed of a gentle slanting surface 114a and a steep slanting surface 114b which has a slanting angle steeper than that of the gentle slanting surface 114a, and the gentle and steep slanting surfaces 114a and 114b are alternately and continuously formed on the opposing surface 112c of the light introduction plate 112.
The liquid crystal display unit 120 is arranged such that a pair of glass substrates 121 and 122, which are opposite to each other through a liquid crystal layer 123 sandwiched therebetween, are joined together through a seal member 124. In the liquid crystal display unit 120, a reflection film 125 and a display circuit 126 are formed so as to be sandwiched between the glass substrate 121 and the liquid crystal layer 123. A display circuit 127 is formed so as to be sandwiched between the glass substrate 122 and the liquid crystal layer 123. Although not shown, the display circuits 126 and 127 include circuits for driving and controlling the liquid crystal layer 123 such as an electrode layer and an orientation film.
In the liquid crystal display device 100, the light introduction plate 112 of the front light 110 is disposed in front of the front surface (upper surface in the figure) of the display region of the liquid crystal display unit 120. The light emitted from the light source 113 is introduced to the light introduction plate 112 through the side end surface 112a thereof, is reflected by the light outgoing surface 112b and the opposing surface 112c, and travels through the inside of the light introduction plate 112. At the same time, the light is reflected by the steep slanting surface 114b, which has the steeper slanting angle, of two slanting surfaces 114a and 114b formed on the opposing surface 112c, and thereby the direction of the light is changed in a direction where the light travels to the light outgoing surface 112b and the light outgoes from the light outgoing surface 112b. The front light 110 illuminates the liquid crystal display unit 120 by the arrangement as described above. Next, the light incident on the liquid crystal display unit 120 passes through the respective display circuits 126 and 127 and the liquid crystal layer 123, reaches the reflection film 125 and is reflected thereby, and returns to the outside of the liquid crystal display unit 120 again. The reflected light passes through the front light 110 and reaches a viewer, and an image displayed on the liquid crystal display unit 120 is visually confirmed by the viewer.
According to the liquid crystal display device 100 arranged as described above, it is possible to display an image even in a dark place by turning on the front light 110, and the image can be displayed brightly at a portion near to the light source 113. However, the image is made darker at a portion thereof which is located farther from the light source 113. In a usual front light, the brightness of the portion of an image displayed at a position about 50 mm apart from the light source 113 is only about one-half that of the portion of the image displayed in the vicinity of the light source 113. Accordingly, it is only a part of the display region of the liquid crystal display device 100 that can realize a sufficient visual property in a dark place. Further, there is also a problem that the existence of the region where an image is displayed dark makes it all the more difficult to view the displayed image.
Accordingly, it is an object of the present invention, which was made to solve the above problem, to provide a surface light emission device capable of obtaining a sufficient quantity of light even at a place apart from a light source, a method of manufacturing the same, and a liquid crystal display device capable of displaying an image the brightness of which is uniform over an entire display region.
In the present invention, a means for solving the above problems is arranged as a surface light emitting device which includes a light source and a light introduction plate having a structure for causing the light from the light source to be incident from a side end surface and causing the light to outgo from a light outgoing surface, wherein a plurality of grooves each composed of a gentle slanting surface and a steep slanting surface having a slanting angle steeper than that of the gentle surface are periodically formed in a stripe shape on the opposing surface of the light introduction plate opposite to the light outgoing surface, and a groove having a larger distance from the light source of the grooves is formed deeper as well as the difference between the depths of adjacent grooves is made larger as they have a distance longer from the light source.
In the surface light emitting device of the present invention, it is preferable that the depth Hn+1 of a (n+1)-th groove from the light source of the plurality of grooves formed in the stripe shape be formed to satisfy a formula Hn+1=Hnxc3x97T/(Txe2x88x92Hn) using the depth Hn of a n-th groove and the thickness T of the light introduction plate.
In the surface light emitting device of the present invention, it is preferable that the depth Hn (xcexcm) of a n-th groove from the light source of the plurality of grooves formed in the stripe shape be shown by a formula Hn=an2T+H1 using the number n of the groove, a coefficient a, the depth H1 ( greater than m) of a first groove, and the thickness T (xcexcm) of the light introduction plate and that the coefficient a is from 1.0xc3x9710xe2x88x928 or more to 5.0xc3x9710xe2x88x928 or less.
In the surface light emitting device of the present invention, it is preferable that the depth Hn (xcexcm) of a n-th groove from the light source of the plurality of grooves formed in the stripe shape be shown by a formula Hn=(an2+bn)xc3x97T+H1 using the number n of the groove, coefficients a and b, the depth H1 (xcexcm) of a first groove, and the thickness T (xcexcm) of the light introduction plate and that the coefficient a is from 1.0xc3x9710xe2x88x928 or more to 5.0xc3x9710xe2x88x928 or less and the coefficient b is from 1.0xc3x9710xe2x88x926 or more to 3.0xc3x9710xe2x88x926 or less.
In the surface light emitting device of the present invention, it is preferable that the depth Fn of a n-th groove from the light source of the plurality of grooves formed in the stripe shape be shown by (Hn+d) which is the sum of an effective portion Hn of the depth Fn that contributes to outgoing of light and an ineffective portion other than the effective portion Hn and that the depth Fn+1 of a (n+1)-th groove from the light source be shown by a formula Fn+1=((Fnxe2x88x92d)xc3x97T/(Txe2x88x92Fn+d))+d using the depth Fn of the n-th groove, the thickness T of the light introduction plate, and the ineffective portion d.
In the surface light emitting device of the present invention, it is preferable that the reflection surface of the light introduction plate have flat portions formed between the grooves which are adjacent to each other.
In the surface light emitting device of the present invention, it is preferable that the reflection surface of the light introduction plate have flat portions formed on the bottoms of the grooves.
In the surface light emitting device of the present invention, it is preferable that the areas of the flat portions be formed so as to be gradually smaller along a light introducing direction.
In the present invention, a means for solving the above problems is arranged as a method of manufacturing a surface light emitting device, which includes a step of producing a casting mold by flatly machining one surface of a casting mold substrate at a predetermined pitch and by forming a plurality of grooves in a stripe shape to the step portions formed on the surface of the casting mold substrate by the flat machining at the pitch; a step of producing a metal mold from the casting mold; a step of producing a light introduction plate by injection molding using the metal mold; and a step of disposing a light source to the light introduction plate, wherein a groove having a larger distance from the light source of the plurality of grooves is formed deeper as well as the difference between the depths of adjacent grooves is made larger as they have a distance longer from the light source.
In the present invention, a means for solving the above problems is arranged as a method of manufacturing a surface light emitting device, which includes a step of producing a metal mold by flatly machining one surface of a metal mold substrate at a predetermined pitch and by forming a plurality of grooves in a stripe shape to the step portions formed on the surface of the metal mold substrate by the flat machining at the pitch; a step of producing a light introduction plate by injection molding using the metal mold; and a step of disposing a light source to the light introduction plate, wherein a groove having a larger distance from the light source of the plurality of grooves is formed deeper as well as the difference between the depths of adjacent grooves is made larger as they have a distance longer from the light source.
In the method of manufacturing the surface light emitting device of the present invention, it is preferable that the depth Fn of a n-th groove from the light source of the plurality of grooves be shown by (Hn+d) which is the sum of an effective portion Hn of the depth Fn that contributes to outgoing of light and an ineffective portion other than the effective portion Hn which is the sum of an effective portion Hn of the depth Fn that contributes to outgoing of light and an ineffective portion other than the effective portion Hn and that the grooves be formed such that the depth Fn+1 of a (n+1)-th groove from the light source satisfies a formula Fn+1=((Fnxe2x88x92d)T/(Txe2x88x92Fn+d))+d using the depth Fn of the n-th groove, the thickness T of the light introduction plate and the ineffective portion d.
In the method of manufacturing the surface light emitting device of the present invention, it is preferable that the depths of the plurality of grooves be formed such that the depth Hn (xcexcm) of a n-th groove from the light source of the grooves satisfies Hn=an2T +H1 which is shown using the number n of the groove, a coefficient a from 1.0xc3x9710xe2x88x928 or more to 5.0xc3x9710xe2x88x928 or less, the thickness T (xcexcm) of the light introduction plate, and the depth H1 (xcexcm) of a first groove.
In the method of manufacturing the surface light emitting device of the present invention, it is preferable that the depths of the plurality of grooves be formed such that the depth Hn (xcexcm) of a n-th groove from the light source of the grooves satisfies a formula Hn=(an2+bn)xc3x97T+H1 which is shown using the number n of the groove, a coefficient a from 1.0xc3x9710xe2x88x928 or more to 5.0xc3x9710xe2x88x928 or less, a coefficient b from 1.0xc3x9710xe2x88x926 or more to 3.0xc3x9710xe2x88x926 or less, the thickness T (xcexcm) of the light introduction plate, and the depth H1 (xcexcm) of a first groove.
In the method of manufacturing the surface light emitting device of the present invention, it is preferable that the grooves be formed such that flat portions are formed between the grooves which are adjacent to each other.
In the method of manufacturing the surface light emitting device of the present invention, it is preferable that the flat portions be formed such that they have a smaller area as they have a larger distance from a side where the light source is disposed.
In the present invention, a means for solving the above problems is arranged as a liquid crystal display device including the surface light emitting device.